This invention relates to spacer washers for disposing between a surface from which a threaded bolt or stud element extends and a nut threaded onto the element.
In this specification, which concerns the disposition of an internally threaded nut on an externally threaded elongate element, the latter may be a bolt, having a separate head passing through apparatus, e.g. flanges, or a stud held by threaded interengagement with the apparatus, and for simplicity and clarity of description, the term "bolt" is used hereinafter as including a stud in the alternative, unless the context indicates otherwise.
The invention is particularly concerned with spacer washers for use with large diameter nut and bolt combinations employed to exert large closing forces on apparatus, such as turbine casings, in circumstances where each nut and bolt combination is one of a plurality disposed in close proximity and with limited accessibility.
Referring to FIGS. 1(a) and 1(b) which show in plan and elevation views respectively a typical section of turbine apparatus 10 in which casing parts having peripheral flanges 12, 13 clamped to each other by means of closely pitched nut and bolt assemblies, 14.sub.1, 14.sub.2 . . . , the clamping force results from the retention of tensile stress in the bolt elements 15.sub.1, 15.sub.2 by means of nut 16.sub.1, 16.sub.2 abutting the flange 12 by way of so-called spherical washer assemblies 17.sub.1, 17.sub.2 which compensate for absence of true perpendicularity of any elements with respect to the flange surface. It is a common feature of such installations that the tensile stress to be retained in the elements must be both large and precisely defined and, furthermore, it is common practice that such tensile stress is induced during installation by stretching the element by way of a temporarily attached, annular hydraulic bolt tensioner rather than by torquing the nut, the bolt tensioner thereafter being removed.
Such annular hydraulic bolt tensioners are well known per se, for example, as described in patent specification GB 1590131. Essentially such a tensioner comprises an annular piston and cylinder arrangement which attaches to, and surrounds, the element and induces tensile stress therein, with consequential stretching of the element, by relative axial displacement of the piston and cylinder. Whilst the tensioner is providing the stress, the nut is run along the element into abutment with the spherical washer with relatively little resistance such that when hydraulic pressure is relieved the tensile stress is maintained by the nut at an accurately predetermined residual level. To react the force exerted by the tensioner on the element into the flange, the tensioner includes a bridge which surrounds or straddles the nut whilst permitting it to be run along the element by way of an access slot in the bridge housing.
It will be seen that not only can the residual tensile stress bend be more accurately defined than when the nut is torqued by overcoming friction, but the tensioner in general may be employed in confined spaces where there is no access for torquing tools.
Situations may arise, as in FIG. 1 wherein it is desirable to have the nut or bolt combinations of such close pitch to each other and/or so close to the casing part that the area of contact between the nut and washer and between washer and flange, which for convenience in this specification is called the footprint of the nut, is all that is available for each nut and bolt pair. However, in order to provide an area of flange for supporting a tensioner bridge straddling the nut during installation or removal, it has been necessary to compromise in terms of having a greater spacing pitch and/or using smaller diameter bolts or nuts than is desirable.
Such situation may be exacerbated, as shown in FIG. 1, by the flange surface being stepped as shown at 10.sub.1 and partially enclosed by other parts, of the casing or flange.
It is an object of the present invention to provide a spacer washer suitable for such a bolted joint arrangement that facilitates easier and improved bolting, and a bolt joint arrangement including such spacer washer.
According to one aspect of the present invention, a spacer washer is provided for a bolted joint of the type in which a threaded bolt or stud element, extending from apparatus is held in tension by means of a nut carried by the element and bearing against the apparatus by way of a spacer washer, having been put into tension by hydraulic bolt tensioning means reacting against the apparatus by way of a bridge straddling the nut, the spacer washer including an elongate body having (i) a through-aperture coaxial with the longitudinal axis of the body and of uniform diameter to provide a clearance fit for the body over a said element, (ii) at one foot end, a body wall of substantially uniform thickness corresponding to the footprint of a said nut, and (iii) at the other head end, in a first direction perpendicular to, and through, the longitudinal axis, a body wall thickness corresponding to the body wall thickness of the foot end and in a second direction, generally orthogonal to the first direction, a body wall thickness greater than that in said first direction, said body head end providing a support face for the nut coextensive with the footprint of the nut and in at least said second direction, a support region of such hydraulic tensioner bridge radially outwardly of the nut footprint.
According to a second aspect of the present invention a bolted joint is provided which includes a threaded bolt or stud element extending from apparatus, a spacer washer as described above, and disposed with said foot end abutting the apparatus and the element extending through said through-aperture, and a nut, threaded onto the element and in abutment with the head end of the spacer washer, holding the element in tension by reaction with the apparatus by way of the spacer washer.
An embodiment of the present invention will now be described by way of example with reference to the accompanying drawings.